Objectives: 1. To examine the effects of K rates on rice growth
and soil extractable K,
2. To compare crop response to K under
different organic material management.

Soil: Orthoxic tropudult (FCC:Ceak) cleared in 1982 with
bulldozers never cropped until this experiment in 1934

Treatments: Treatments are an incomplete factorial set out in a
randomized complete block design with 3 replications.
Six rates of K, as KC1 fertilizer, were applied to
plots having previous crop residues removed: 0, 20,
40, 80, 120, and 240 kg K/ha. Three rates (0, 40 and
80 kg K/ha) were applied to plots having residue from
the pruious rice and soybean crops and also weeds
(primarily a grass, tentatively identified as Paspalam
sp., which had grown during the dry season)
incorporated. The 40 kg K/ha plus crop residues
treatment was formerly a 10 kg K/ha residues removed
treatment in the previous year. Green manure
treatments of 10 t fresh Calapogonium were made (as
per last year) in combination with two levels of K
fertilizer, 0 and 80 kg K/ha.

Crop History: Upland rice, cv. Sentani, was planted on Sep:. 28,
1985, with a blanket application of the following
fertilizers (in kg/ha): 100 TSP, 125 kieserite and 33
urea applied twice (at 1 and 2 months after planning).
Curaterr 3G (carbofuran) was applied at planting at a
rate of 30 kg/ha. Several sprays of Sevin and
Diazinon were made during grain filling. Harvest
occurred on January 19, 1986, 114 days after planting.

Results: As preparations were being made to replant this
experiment after an unusually wet dry season, it was noticed that
weed growth (primarily a grass tentatively identified as a
Paspalam sp.) was related to treatment. This weed (grass) growth
was cut and weighed, with results in Figure 1. With previous crop
residues removed, grass dry weight had a linear correlation of Y=
0.87 + 0.019 applied K and an R =0.97. Returning crop residues

gave higher yields at 0 and 80 kg K/ha while green manure
applications gave the highest yields at these 2 rates.

Grain yields of rice were quite high (Figure 2), due to very
wet weather and low pest pressures. Plateau yields of 3.5 t/ha
were 6-7 times greater than yields without KC1 fertilizer and
with residues removed. A rate of 80 kg K/ha was sufficient to
reach the plateau. Crop response to fertilizer K with residues
returned was not as strong, primarily because yields at 0 K were
much higher (above 2 t/ha). It seems evident that recycling of
crop residues in these soils is very important, especially at low
rates of K fertilizer. Figure 2 illustrates that simply returning
crop residues (with no KC1 applications) gave equivalent yields
to those achieved with 20 kg K/ha when crop residues were
removed. Green manure applications were even more effective at
increasing grain yields when no K fertilizer was applied.

Table 1 shows the effects of treatments on blast
(Pyricularia oryzae Cay.) incidence at both 54 and 82 days after
planting. Potassium fertilizer was very effective at decreasing
blast scores, and less blast occurred when residues were
returned. Application of green manure apparently was as
effective as KC1 in imparting disease resistance to the rice.

The response curve for rice straw yields from both 19E5 and
1986 are shown in Figure 3. Although yields were higher in 1986,
response for both years was similar, with plateau yields being
achieved at 80 kg K/ha. Green manure treatments are plotted to
indicate the total amount of K (organic + fertilizer) being
applied (Figure 3), and they appear to coincide with the response
curve for KC1 alone. Evidently, the major reason green mar-re
was so effective at increasing yields in this trial was due to
the amounts of K being applied organically.

Mehlich I extractable K correlated very poorly with grain
yield (R =0.30), and further work is in progress to address the
issue of soil testing for K in these soils.

Table 1. Effect of K rates,
of blast incidence

reidue management and
o cv. Sentani.

green nanure

Treatment
inorganic organic*
kg K/ha

0
20'
40
80
120
240

IR
IR
IR

IR+GM
IR+GM

Blast Rating (0-9)
54 DAP 82 DAP

3.7
2.7
2.0
2.0
2.0
2.0

2.7
2.0
2.0

2.0
1.7

6.0
5.7
4.0
4.0
3.7
3.7

4.7
4.0
4.0

4.7
3.7

* RR=remove residues, IR=incorporate residues, GM=green manure

* KCI only
R- residue returned
G green manure

0 40 80
Applied

Figure 1.

120
*K (kg/ha)

240

Response of a mixture of primarily grass
(probably Paspalum sp.) and a few other weeds
to applied K, crop residue incorporation and
green manure applications.